At the present time, the majority of IC chips are packaged in a dual-in-line package configuration, with packages having 4 to 64 leads with 0.100 inch external spacing. The dual-in-line package has generally proven inexpensive and reliable in configurations having up to 24 leads. However, above 24 leads, the dual-in-line package becomes less desirable. This is because, generally, the internal lead density increases with lead count and it becomes very difficult to accurately maintain sufficiently precise internal lead dimensions for dual-in-line packages above 24 leads as required by subsequent semiconductor assembly.
The IC industry is, however, moving toward higher and higher lead count applications with packages of 68 to 164 leads on the horizon. To package the larger devices, workers have begun to use packages called chip carriers. The chip carrier has leads or contacts on all four sides of the package (when viewed from above) and the contact or lead spacing is usually 0.050 inch or smaller, with some applications using a lead spacing as small as 0.020 inch. Because of the reduced lead spacing as compared to the dual-in-line package, the chip carriers are generally soldered directly to the surface of printed circuit boards rather than being mounted through holes in the printed circuit board as for the dual-in-line packages, since printed circuit boards cannot accurately maintain 0.050 inch hole spacing.
Although the chip carrier is mounted directly to the surface of the circuit boards, it is still desirable to have complient leads eminating out from the package body. In such a leaded chip carrier configuration, the leads serve to absorb the stress caused by the mismatch of thermal coefficients of expansion between the IC package and the circuit board. Unfortunately, the leadframes needed for such high lead count packages are very costly to produce.
Chip carrier leadframes are machine stamped as a series of units 100 as shown in FIG. 1 in a continuous metal strip. After stamping, the strip is rolled onto a reel containing a desired number of leadframes. The reel is then unrolled and processed through a first masking operation to mask all areas except the internal fingers 110. After masking, aluminum is vapor deposited on the exposed internal lead fingers 110. The first mask is then removed leaving selectively vapor deposited aluminum on the internal lead fingers 110. In some applications, gold plating is also required on the external leads 120. This is done by masking the leadframes 100 a second time, gold electroplating the exposed external leads 120, and then removing the second mask. The continuous metal strip is then cut into individual units 100 for subsequent package manufacturing.
Unfortunately, the precision stamping tools needed for producing the units 100 are very expensive, require a relatively long time to fabricate, and have significant maintenance and repair costs due to their complexity. In addition, the stamping yields are often low because of the complex nature of the stamping tools. Also, vapor depositing aluminum is an expensive operation and the resulting deposited aluminum is too soft, too thin and too weakly bound to the underlying metal for many applications.
Finally, the use of two separate masks for selectively applying aluminum and gold is quite expensive.